Barack Obama began an uphill struggle on Monday, to persuade Congress of the need for US military action in Syria. The president began by reaching out to hawkish Republicans, such as senators John McCain and Lindsey Graham, in the hope of building a consensus before key votes take place next week.

Three scenarios are apparent when assessing how the outcomes of deliberations in the House and the Senate will affect Obama:

Yes votes

The most simple route to war for Obama would be to secure authorisation from the House and the Senate within the next two weeks. Despite the wide-ranging scepticism among politicians at present, the president has at least been congratulated for his surprise decision to consult lawmakers.

In the House, Obama is hoping that a decision by speaker John Boehner not to join early Republican criticism is a sign that he is also open to a deal. The House intelligence committee chairman, Republican Mike Rogers, has predicted that Congress will "rise to the occasion".

The White House insists that its military advisers have calculated that a delay of a week or two will not have much impact on the outcome of any strikes. Should both houses approve the strikes, Obama would be able to proceed knowing that Congress was equally invested in the success of the mission.

Mixed votes

A equally likely scenario is that the verdict from Congress will be split, with the Democrat-controlled Senate remaining loyal to the president but the Republican-dominated House choosing to once again defy his wishes.

The Republican senator Rand Paul, an opponent of the White House, puts the chances of the House agreeing to military action, even with a much-revised draft authorisation, at no better than 50/50 – although even he concedes that a yes vote is likely in the Senate.

Such a split vote would put Obama in the nightmare position of probably having to proceed, but with only limited Congressional backing.

Secretary of state Kerry has been quick to remind Washington that there is no legal requirement for the president to have both chambers vote in favour – but it would be hard to ignore a substantial "no" vote in the House.

No votes

It is still possible, that Obama will be unable to persuade either chamber to go along with his plan.

Despite a narrow majority in the Senate, the White House has not been able to rely on universal support from Democrats, even on issues such as gun control. On the issue of strikes against Syria, a unanimous rejection could deal a fatal blow to Obama's second-term credibility.

He might at least be able to follow the path taken by David Cameron when the British prime minister was faced with a similar rejection last week, and take the moral high ground: blaming lawmakers for failing the Syrian people while secretly breathing a sigh of relief for having one of the trickiest foreign policy decisions of recent years taken out of his hands.